The semiconductor integrated circuit (IC) industry has experienced exponential growth over the last few decades. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased.
In order to form an integrated circuit device in a small area, vertical transistors are developed. In a typical vertical transistor, a conducting channel is provided by a silicon nanowire which perpendicular to a plane of an underlying semiconductor substrate. The vertical channel is surrounded by a three-dimensional gate structure. The three-dimensional gate structure can be referred to as vertical-gate-all-around (VGAA) configuration.